This invention relates to improvements in disc refiners providing for a simple, effective and controlled venting of steam produced in a disc refining operation. It is particularly advantageous in application to double revolving disc refiners and it will be so described by way of illustration. However, it is to be understood, and should become obvious, that the application of the invention is not so limited, and such is not intended.
The invention is directed to minimizing problems long evidenced in use of the prior art disc refiners. It has been found that substantially all the power applied in a disc refiner is converted to heat, developed as material is reduced between the relatively opposed refining surfaces of the refiner discs. A major portion of this heat functions to convert the moisture or liquid which is present to steam. Another portion of this heat is absorbed by the material being refined. The degree of heat developed is the source of the problems to a solution of which the present invention is directed. To understand the problem, consider if you will that in the operation thereof a disc refiner will generate heat capable of producing about a ton of steam at 212.degree. F. per ton of wood or fibrous materials passed between the refiner discs when power is applied at the level of 60 H.P. days per ton of the material delivered in chip form at 15% consistency. Such a level of operation, or one with an even higher "effective" power input, is desirable to enable higher consistency refining and to obtain high quality products or, in the alternative, to enable the obtaining of the same quality products with a lesser power input. However, due to the considerable magnitude of steam developed, it has been found difficult if not impossible to operate a disc refiner under such conditions for any period of time without serious and undesirable consequences. As a matter of fact, experience has shown that when utilizing prior art disc refiner equipment one cannot afford normally to put more than about 50 H.P. days per ton into any disc refining operation and expect any degree of operating efficiency.
The nature of the operational problems solved by the present invention is exemplified as follows.
With an overabundance of steam, while part of the steam developed will exit with the refined material and in such case merely represent a loss of energy, the steam remaining in the disc refiner will, in conventional equipment, often times create a blowback condition. The import of such a condition is that the steam will move to interfere with the inflow to the refiner of the material to be refined. Of particular concern in the production of fibrous pulp is the fact that the interference of the steam will cause a nonuniform rate of material infeed and consequently a nonuniform quality of the end product issuing from the refiner. In many instances it has been found that the quantity of steam produced between the refining surfaces of a disc refiner in the course of its operation is so great that there is not only an interference with infeed but in fact a development of sufficient pressure in the eye of the refiner to completely block incoming material from passage to and between the refining surfaces of the refiner discs. This last can occur even though a positive displacement infeed device is provided in the feed passage leading to the inlet to the refiner. As a matter of fact, the restriction of the infeed passage by the positive displacement feed device can contribute to the buildup of the pressure created in the eye of the refiner. In any case, the foregoing presents a concise description of the problems in the prior art which can be alleviated by usage of embodiments of the present invention.
It has been the practice in the prior art to offset some of the problems above enumerated by the introduction of excess water with the material being refined, the purpose being to quench the steam. While this has helped to some degree, it is not only wasteful but it does cause an undesirable reduction in the consistency of the material being refined and a consequent reduction of the quality level of the resultant end product.